Evaluation of an Ensemble Dispersion Calculation

Roland R. Draxler

Journal of Applied Meteorology,

**Abstract -** A Lagrangian transport and
dispersion model was modified to generate multiple simulations from
a single meteorological data set. Each member of the
simulation was computed assuming a ± one grid point shift in
the horizontal and a ±250 m shift in the vertical of
the particle position with respect to the meteorological
data. The configuration resulted in 27 ensemble
members. Each member was assumed to have an equal
probability. The model was tested by creating an ensemble of
daily average air concentrations for three months at 75 measurement
locations over the eastern half of the United States during the
Across North America Tracer Experiment (ANATEX). Two generic
graphical displays were developed to summarize the ensemble
prediction and the resulting concentration probabilities for a
specific event: a "probability exceed" (PE) plot and a
"concentration probability" (CP) plot. Although a cumulative
distribution of the ensemble probabilities compared with the
measurement data was favorable, the resulting distribution was not
uniform. This was attributed to release height sensitivity.
The trajectory ensemble approach accounts for about 41% to 47% of
the variance in the measurement data. This residual uncertainty is
caused by other model and data errors not included in the ensemble
design.

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